Determination of the Jet Energy Resolution and Measurements of Jet Substructure and the Top Quark Mass in Decays of Boosted Top Quarks at CMS

Abstract

This thesis presents two analyses using data recorded by the CMS detector in proton-proton collisions at the LHC. First, a determination of the jet transverse momentum resolution scale factors is performed, followed by the measurement of the jet mass in hadronic decays of boosted top quarks and by an extraction of the top quark mass.

The first analysis determines the jet transverse momentum resolution scale factors for data collected during the LHC Run 2 and Run 3 data-taking periods. The width of the jet energy response is calibrated in simulation to match the width in data, exploiting the transverse momentum balance in QCD dijet events. In this thesis, this method has been extended to cover a jet transverse momentum from 200 GeV to 2000 GeV. A modified correction method for additional jet activity is introduced, refining the uncertainty treatment and the reliability of the calibration. The compatibility of simulation to data is studied, particularly in low transverse momentum ranges which are constrained due to the additional jet activity. The final calibration for data collected at a center-of-mass energy of 13 TeV has been performed, covering the years 2016, 2017, and 2018, corresponding to an integrated luminosity of 138 fb^-1. Moreover, the first calibrations for data collected at 13.6 TeV is presented for 2022 and 2023 corresponding to 62 fb^-1.

The second analysis presents the measurement of the differential top quark pair production cross section as a function of the jet mass in decays of boosted top quarks, using data collected at 13 TeV corresponding to 138 fb^-1. At high momenta, the decay products of top quarks are highly Lorentz boosted and can be reconstructed within a single jet, which require a detailed understanding of the jet substructure. The reconstruction of the hadronic top quark decay in a single jet provides the opportunity to explore new energy regimes. Moreover, the jet mass distribution can be compared to analytic calculations, allowing for the extraction of the top quark mass in a well-defined mass scheme. In this work, the focus is set on the dominant uncertainties of an earlier analysis result, the calibration of the jet mass scale and the modeling of the final state radiation. The uncertainty of the jet mass scale is constrained by calibrating the jet mass scale to the reconstructed W boson mass. A refined modeling of the final state radiation is introduced, based on the N-subjettiness. This reduces the uncertainty of the jet mass scale by a factor of three and minimizes the final state radiation modeling uncertainty, making it a negligible source of uncertainties. The jet mass is unfolded to particle level and used to extract the top quark mass with (173.06 +- 0.84) GeV.